Image sensing devices, such as a charge-coupled device (CCD), are commonly found in such products as digital cameras, scanners, and video cameras. These image sensing devices have a very limited dynamic range when compared to traditional negative film products. A typical image sensing device has a dynamic range of about 5 stops. This means that the exposure for a typical scene must be determined with a fair amount of accuracy in order to avoid clipping the signal. In addition, oftentimes the scene has a very wide dynamic range as a result of multiple illuminants (e.g. front-lit and back-lit portions of a scene). In the case of a wide dynamic range scene, choosing an appropriate exposure for the subject often necessitates clipping data in another part of the image. Thus, the inferior dynamic range of an image sensing device relative to silver halide media results in lower image quality for images obtained by an image sensing device.
An increase in the dynamic range of an image sensing device would allow images from digital cameras to be rebalanced to achieve a more pleasing rendition of the image. Also, increasing the dynamic range of an image sensing device would allow for more pleasing contrast improvements to the image, such as is described by Lee et al. in commonly assigned U.S. Pat. No. 5,012,333 issued Apr. 30, 1991.
U.S. Pat. No. 6,040,858 issued Mar. 21, 2000 to Ikeda provides a complete description of the problem of the limited dynamic range of image sensing devices. In addition, Ikeda describes methods of extending the dynamic range of an image sensing device by utilizing multiple image signals, each with different responses to exposure. These multiple signals are combined by using thresholds which determine which signal is of higher quality at each position in the image signal to form an image signal having extended dynamic range. Ikeda improves upon these methods by describing a method by which these thresholds are determined for each color.
In addition to the dynamic range limitations associated with common image sensing devices, another problem is that color encodings associated with most common digital image storage formats also have a limited dynamic range. Typically, users of digital imaging devices, such as digital cameras, are accustomed to receiving digital images that are stored in a color encoding that will produce a pleasing image when displayed directly on a typical CRT monitor. This arrangement is convenient for many typical work-flows, and is desirable in many cases to maximize compatibility and interoperability. CRT monitors have a substantially limited color gamut relative to the color gamut of many common image sensing devices. (The color gamut of an imaging device refers to the range of colors and luminance values that can be produced by the device. The dynamic range is one aspect of color gamut relating to the range of luminance values that can be produced by the device.) The process of converting the color values captured by an image sensing device to those appropriate for display on a particular output device is often referred to as “rendering.” The rendering process will typically result in a significant loss of image data corresponding to areas of the scene having colors outside the color gamut of the rendered space. Thus, even if an image sensing device with an extended dynamic range were available, the extra dynamic range may not be enjoyed, appreciated, or even noticed by the user if the image must be stored in a limited color gamut storage space before the extended color gamut image data can be used to form an improved image.
Thus, there exists a need to improve upon the method of the prior art in order to improve the dynamic range of an image sensing device and to allow the additional dynamic range of the device to be stored in a form useful to consumers. Specifically, there exists a need to produce an extended dynamic range image signal with a single image sensing device and a single image signal, and to represent that image signal in a standard form compatible with common image work-flows.